Zeolitic aluminosilicates are characterized by the general empirical formula (oxide formula): EQU x Cat.sub.2/n O . Al.sub.2 O.sub.3 . y SiO.sub.2 . z H.sub.2 O
in this oxide formula, Cat denotes a cation of the valence n, which is exchangeable with other cations, x denotes a number from 0.2 to 1.5, preferably approximately 1, y is a number larger than 1.5. The water content (z) depends on the degree of dryness. The above formula includes a plurality of substances which differ frequently only slightly in their chemical composition, but quite considerably in their structure and properties. In addition to the chemical composition, the X-ray diffraction diagram is generally used in crystalline types for identification.
Synthetic zeolites are of great technical importance and are used as cation-exchangers, particularly for softening water, as catalysts in chemical processes, as drying agents, separating agents, and sorption agents for solvents and gases, and as heterogeneous inorganic builders in washing and cleaning agents. Depending on their use, different types, degrees of dryness and purity are required.
Of particular technical importance are the two molecular sieve types A and X, which are also called by different names in the literature. Molecular sieve type A has the summation formula: EQU 1.0 .+-. 0.2 Cat.sub.2/n O . 1.0 Al.sub.2 O.sub.3 . 2.0 .+-. 0.5 SiO.sub.2 . 0 to 6 H.sub.2 O
and molecular sieve type X has the silicate enriched summation formula: EQU 0.9 .+-. 0.2 Cat.sub.2/n O . 1.0 Al.sub.2 O.sub.3 . 2.5 .+-. 0.5 SiO.sub.2 . 0 to 8 H.sub.2 O.
it is customary to prepare the molecular sieves first in their sodium form and to transform them then into other forms by cation-exchange (corresponding to the designations NaA and NaX, respectively, for the sodium forms). The X-ray diffraction diagram of molecular sieve NaA and the X-ray diffraction diagram of molecular sieve NaX are given in U.S. Pat. Nos. 2,882,243 and 2,882,244.
Another zeolitic aluminosilicate of increasing technical importance is the cubic molecular sieve P with a high silicate content, which is also called among others "Zeolite Pc" or "Molecular sieve B". The X-ray diffraction diagram is given, for example, in D. W. Breck, "Zeolite Molecular Sieves", New York, 1975, p. 365, and U.S. Pat. No. 3,008,803.
Two principally different synthetic methods are employed in the preparation of molecular sieves. The first method consists in crystallizing alkali metal aluminosilicate gels, which are formed by mixing preferably completely dissolved alkali metal aluminate and alkali metal silicate solutions with an excess of alkali in the batch. This way it is possible to obtain cation-exchanging products which contain less than 100 ppm impurities. However, this method has the disadvantage that it is relatively elaborate, particularly if we consider that the technical products used as the aluminate component and the silicate component must first be prepared.
The second synthetic method for the preparation of zeolitic aluminosilicates consists in crystallizing mineral aluminosilicate components which have been activated, if necessary, from a highly alkaline solution.
By using mineral aluminosilicates which are found abundantly in the earth's crust, the preparation of the zeolitic aluminosilicates can be considerably simplified and reduced in cost. Such mineral aluminosilicates which can be used are clays (such as attpulgite, bauxite, diaspore clay), vulcanic vitreous lavas (such as obsidian, perlite, pitchstone, pumice), other non-magmatic, non-kaolinic stones (such as andalusite, albite, cyanite, nepheline). These methods are described in DAS No. 1,191,346. Particularly suitable are minerals of the kaolinite group (kaolinite, nacrite, dickite, and halloysite), hereafter called "kaolin".
These minerals, preferably kaolin and pumice, can be converted to synthetic zeolites of varying purity, mostly after activation, with suitable molar ratios: EQU Na.sub.2 O : Al.sub.2 O.sub.3 : SiO.sub.2 : H.sub.2 O
adjusted in the charge, by hydrothermal treatment in a highly alkaline solution.
Customary kaolins consisting primarily of alumina minerals of the general chemical composition: EQU Al.sub.2 O.sub.3 . 2 SiO.sub.2 . 2 H.sub.2 O
can be converted practically completely (after activation) by a brief hydrothermal treatment with aqueous sodium hydroxide at temperatures above room temperature, preferably above 70.degree. C, depending on the impurities, to the cation-exchanging aluminosilicate type NaA.
In the conversion of kaolin to molecular sieves with a high silicate content, such as NaX or Pc, it is necessary to "resilicate" the charge by adding additional, preferably soluble reactive silicate components, such as waterglass solution, crushed glass, sodium metasilicate, precipitation silica, and the like, corresponding to the desired molar ratios in the charge.
The activation can be effected by first calcining kaolin at 550.degree. to 900.degree. C, whereby it is converted to an anhydrous, destructured and therefore X-ray amorphous metakaolin, Al.sub.2 O.sub.3 . 2 SiO.sub.2. In modifications of this process, the thermal activation is effected in the presence of alkali and, optionally, silicate at temperatures of 250.degree. to 500.degree. C. Kaolin can also be activated, however, by thorough grinding or by acid treatment.
Aluminosilicate zeolites type P of the composition: EQU 1 .+-. 0.2 Na.sub.2 O : 1 Al.sub.2 O.sub.3 : 4.0 .+-. 1.3 SiO.sub.2 : y H.sub.2 O (y = 2 to 7)
preferably: EQU 1 .+-. 0.1 Na.sub.2 O : 1 Al.sub.2 O.sub.3 : 3.6 .+-. 0.8 SiO.sub.2 : y H.sub.2 O (y = 2 to 7)
can be obtained from kaolin, by preparing a mixture of destructured kaolin, sodium hydroxide and silicon dioxide, or a compound which is capable of supplying silicon dioxide under the above-indicated conditions, and water, where the portions of the reaction partners are so selected that the composition of the reaction mixture in the charge is in the range of the molar ratios: EQU 3 to 10 Na.sub.2 O : 1 Al.sub.2 O.sub.3 : 3 to 10 SiO.sub.2 : 100 to 500 H.sub.2 O,
with the proviso that the molar ratio: EQU SiO.sub.2 : Na.sub.2 O
is equal to or greater than 1, and that the subsequent crystallization is carried out preferably under stirring at temperatures above 95.degree. C within 24 hours. The volcanic glasses, on the other hand, are X-ray amorphous from the beginning. A further destructurization is, therefore, not necessary in most cases.
The high temperature treatment in the presence of caustic alkalis (and other components, if necessary) at temperatures preferably below 500.degree. C is a well-known method of additional activation. Since these vitreous lavas usually have a high silicate excess (molar ratio SiO.sub.2 /Al.sub.2 O mostly over 6, in some cases even over 10), the charge must be enriched with aluminate components to obtain the molecular sieves type A and X with the lower silicate content. Pumice is a particularly prevalent volcanic earth. In addition to the principal components Al.sub.2 O.sub.3 and SiO.sub.2 it already contains considerable amounts of alkali metal oxides (up to 10%).
The kaolinic and volcanic aluminosilicates preferably used for conversion into zeolitic molecular sieves always have a considerable content of impurities, such as CaO, MgO, TiO.sub.2, Fe.sub.2 O.sub.3, and similar metal oxides, which is particularly high in pumice.
Of these impurities, iron oxide has a particularly negative effect in the subsequent use of the resulting molecular sieves, e.g., in the use of the products as catalysts in chemical processes. Iron-containing molecular sieves have a destabilizing effect on percompounds which might be contained in a solid mixture or in a suspension, used particularly in textile treatments.
The methods described so far for reducing the iron content of mineral aluminosilicates were not very successful, despite relatively high costs. Thus it has been suggested to treat kaolin in acid solution with reducing agents and then with organic ion exchange resins. The ion exchange resins loaded with iron were removed from the suspension by screening. Such a preliminary acid treatment in cumbersome, particularly when the following conversion of the aluminosilicates to molecular sieves takes place in a highly alkaline solution.